1. You like each other's extreme traits

Which is more often true: "Opposites attract," or "Birds of a feather flock together?"

In general, it's the latter. People really do tend to partner with others who are similar to them and who share their values.

Of course, this is a generalization. It's easy enough for us to think of relationships that work despite differences (e.g., Democrats and Republicans, vegetarians and meat-lovers, extroverts and introverts). Where shared inclinations and values tend to be most important, though, is if one or both partners is very extreme in a certain area.

For example, my spouse and I are both extremely frugal. We drive most other people nuts with our resistance to spending money. Most other potential partners would find either of us difficult to live with because we're on the extreme end of the normal curve in this regard.

Extreme opposite traits can initially attract you to someone — for example, a shy, quiet person attracted to someone who has hundreds of friends and is an expert social networker.

If your partner is warm towards you and other people, that's a good sign. Likewise, if they're emotionally stable and dependable, that's also a good sign. If they're aloof or narcissistic towards others, but nice to you, that's a potential warning sign: Their treatment of others may be more revealing of their true colors.

3. You and your partner have roughly equal 'mate value'

Research shows that love isn't particularly subjective: Even relative strangers are usually fairly reliable in rating someone's "mate value," i.e., desirability. People also usually have a rough idea of their own mate value.

Problems can occur in relationships when the partners have vastly different objective mate value. We've all seen celebrity marriages in which an attractive, wealthy individual marries someone who doesn't appear to even have a job. Having a partner with lower mate value than yours may be initially attractive, but the sheen typically wears off after time (as we see when such celebrity couples break up).

The reality is that wealthy or attractive people tend to marry other wealthy or attractive people. One might imagine that everyone would want a partner with "10/10" mate value, but this is often stressful due to what evolutionary psychologists call fear of " mate poaching" — if your partner is, say, Maria Sharapova or Serena Williams, chances are you're going to be worried about losing your lover to a poacher.

In general, then, having a partner who has roughly equal mate value to you typically makes for the smoothest, most enduring relationship.

[Note: These are principles of evolutionary psychology. They can seem a bit crass in the modern world, but research shows that they are generally true.]

4. Your partner helps you feel good about yourself

In happy relationships, people generally view their romantic partners just a little bit more positively than they see themselves. For example, you may view yourself as a 6 out of 10 in terms of physical attractiveness, but your partner views you as a 7.

Partners viewing with a slight (but not ridiculous) positive bias is a benefit of a well-functioning relationship, and a good sign for its longevity.

According to research, how your partner sees you is most important in three key domains— warmth/trustworthiness; attractiveness/vitality; and status/resources (or potential to achieve them).

How they view things like your singing or artistic ability is generally less important to the overall course of the relationship than their view of you in the three domains most central to mate value (assuming you're not actually a professional singer or artist...).

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